H3PO4 Titration Problem: Determining Molar Concentration and Indicator Choice

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In summary, the conversation discusses titrating an H3PO4 solution with NaOH and determining the molar concentration of the acid. The use of bromophenol blue as an indicator is also questioned. The topic of equivalence points and calculating pH is brought up, and the conversation references an article for accurate formulas. It is determined that the theoretical pH at the first equivalence point is 4.70.
  • #1
yolo123
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Hello Forum,
Another problem again... Please bare with me.

You are titrating an H3PO4 solution of acid. You titrate 20ml of that solution. You have used 15 ml of 0.500 M NaOH at the equivalence point. What is the molar concentration of the acid? Would bromophenol blue (pH 3.0-4.6) be appropriate to use as an indicator? (ie, find the pH)

Please look at my steps. Would you get the same answer as me?

My acid base theory is really wobbly and I need to perfect it. I want to make sure I have a strong foundation, especially because I want to take organic chemistry (but I need to get at least 75% in my current class :S)
 

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  • #2
I am not convinced your calculation of equivalence point is correct. You have a solution of an amphiprotic salt NaH2PO4.

This is about shape of titration curve - is there only one, or two separate equivalence points? Once you know equivalence point pH you should check how much of the H2PO4- is neutralized at this pH. If almost none, that's OK, if too much - there will be no well visible, separate inflection points.

Actually there are two, see the titration curve.

attachment.php?attachmentid=69775&stc=1&d=1400050786.png
 

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  • #3
Is the definition of equivalence point not: moles of titrant=moles of analyte?
If so, then all H3PO4 becomes H2PO4- which will now act as an acid.
 
  • #4
yolo123 said:
If so, then all H3PO4 becomes H2PO4- which will now act as an acid.

It also acts as a base. And pH at the first equivalence point is definitely not 3.94 that you calculated.
 
  • #5
I read the article you linked. But, we never saw those kinds of formulas in class! What would I do to get that 4.70?
 
  • #6
http://www.occc.edu/clvahlberg/documents/chem1215/Unit%204%20-%20Equilibria/phosphorc%20acid%20eq%20points.pdf

These guys did it similarly to me. Are they wrong?
 
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  • #7
yolo123 said:
I read the article you linked. But, we never saw those kinds of formulas in class! What would I do to get that 4.70?

Not sure what the question is - just use formulas derived there. There is even a table that shows how the accurately calculated pH differs form the approximated result calculate with the derived formula exactly for your solution.

yolo123 said:
http://www.occc.edu/clvahlberg/documents/chem1215/Unit%204%20-%20Equilibria/phosphorc%20acid%20eq%20points.pdf

These guys did it similarly to me. Are they wrong?

Unfortunately yes, they are wrong. Theoretical pH at the first equivalence is 4.70.
 
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Related to H3PO4 Titration Problem: Determining Molar Concentration and Indicator Choice

1. What is the purpose of performing a H3PO4 titration?

The purpose of performing a H3PO4 titration is to determine the concentration of phosphoric acid in a solution. This information can be used to calculate the amount of acid or base needed to reach a desired pH in a chemical reaction or to determine the acidity of a substance.

2. What materials are needed to perform a H3PO4 titration?

The materials needed for a H3PO4 titration include a burette, pipette, indicator, standardized solution of a known concentration, and the sample of H3PO4 solution to be titrated. Other materials may also be required, such as a flask, beaker, and funnel.

3. How is a H3PO4 titration performed?

A H3PO4 titration is typically performed by adding a standardized solution of a known concentration to a measured volume of the H3PO4 solution. The solution is slowly added until the indicator changes color, indicating the endpoint of the titration. The volume of the standardized solution used is then used to calculate the concentration of the H3PO4 solution.

4. What is the purpose of using an indicator in a H3PO4 titration?

An indicator is used in a H3PO4 titration to show when the endpoint of the titration has been reached. This is typically indicated by a color change in the solution. The choice of indicator will depend on the expected pH of the H3PO4 solution, as different indicators have different pH ranges in which they change color.

5. What are some sources of error in a H3PO4 titration?

Some sources of error in a H3PO4 titration include inaccurate measurements of volumes, errors in the standardization of the solution used, and the presence of impurities in the H3PO4 solution. It is important to carefully follow the procedure and use precise instruments to minimize these sources of error.

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